The invention relates generally to power supplies and deals more particularly with a circuit which permits a three phase input power supply to operate when one phase drops out, and also provides protection from lightning.
Many types of DC power supplies operate from a three phase power source. Occasionally, one phase drops out. It was known in the prior art to detect when one phase drops out, and in response, shut off the power supply altogether. This solution is not acceptable for critical loads that cannot afford a shut down.
Power supplies also need protection from lightning surges which may dramatically increase the line voltage. It was known to use a zener diode or switch to limit the voltage applied to the power supply input from the line. Capacitance at the input to the power supply is also effective in shunting short surges; however the capacitance reduces the power factor of the power supply during normal operation. A very low power factor may be objectionable to the power company and require heavier gauge wiring at the site where the power supply is being used.
Accordingly, a general object of the present invention is to provide a circuit which permits a three phase input power supply to operate when one phase drops out.
Another general object of the present invention is to provide such a circuit which also protects the power supply against lightning surges on the power line.
Another object of the present invention is to provide circuits of the foregoing types with high power factor during normal operation.